<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>固定定位</title>
    <style>
        *{margin: 0;
        padding: 0;}
         .box1
         { height: 100000px;background-color: red;}
         .box2
         { width: 200px;height: 20px;background-color: blue;position: fixed;left: 50%;margin-left: -100px;}
    </style>
</head>
<body>
    <div class="box1">
        <div class="box2"></div>
    </div>
</body>
</html>